How are ZIP Codes Assigned?

There are more than 40,000 ZIP codes in the United States, and there’s a system as to how they’re designated.

It’s hard to imagine how mail could function without them, but ZIP codes weren’t always around. Before 1963, cities simply had “postal zones” in 124 of the nation’s most populated areas. But when mail started to boom, in order to meet the growing demand, the U.S. Postal Service decided to add five numbers to addresses to make mail more efficient as a part of a Zone Improvement Plan (the ZIP in ZIP code stands for Zone Improvement Plan). Around the same time, the Postal Service familiarized people with two-letter state abbreviations to be used in addresses to shorten state names like Pennsylvania (PA) and California (CA).

In order to promote the idea of ZIP codes, “Mr. Zip” appeared as the cartoon mascot for the campaign, touting “Put ZIP in your mail!” While Mr. Zip is obviously a fictional figure, a Philadelphia postal inspector named Robert Moon is widely considered to be the real-life version and the father of the ZIP code even though the Postal Service refused to give him sole credit for the invention. Moon first submitted his “coding addresses” idea in 1944 and didn’t hear back. Then, after the third time he submitted it, the USPS adopted it but credited a committee with its development.

On July 1, 1963, the Postal Service launched the use of ZIP codes, and today, they mean more than just where people are—they are often indicative of who people are, with some ZIP codes denoting wealth. And they’re not just reserved for cities. Anywhere that receives a high volume of mail can receive a ZIP code, including government agencies, universities, and businesses.

ZIP Code Zoning

Typically, each state starts with a certain number or number range. For example, Montana always starts with “59” based on its zone. On the other hand, Pennsylvania ZIP codes can start with anything between “150” and “196.” To see the ZIP code zones across the country, click here to view the map.

What do the digits indicate?

The first number indicates a ZIP code zone. Here’s a look at the 9 zones:

The first three digits of the ZIP code indicate the sectional center facility (SCF) or a central mail processing facility that serves one or more three-digit ZIP codes. There are sectional center facilities all over the United States.

For example, if a ZIP code starts with a 7, you know it’s going to one of four states: Louisiana, Arkansas, Oklahoma, or Texas. Then, the second two numbers will tell you what state. If it’s between 716 and 729, then you know it’s going to be in Arkansas. So 716 would be connected with Pine Bluff, AR.

The last two digits in a ZIP code are how the mail is actually sorted to specific postal offices. The “plus-four” add-on codes that were launched in 1983 are supposed to identify a specific geographic location within the five-digit area. Although the full nine-digit ZIP codes are not required in order to send mail, they are increasing in everyday use.

The Bottom Line

The ZIP code is not just a randomly generated number. There is meaning and organization behind it, which is why it’s a system that’s lasted more than 50 years.